CONODONTS, SEQUENCE STRATIGRAPHY AND THE DROWNING OF THE SAN JUAN CARBONATE PLATFORM IN THE ORDOVICIAN OF THE ARGENTINE PRECORDILLERA

CARRERA, MARCELO G.; FENOGLIO, FEDERICO; ALBANESI, GUILLERMO LUIS; VOLDMAN, GUSTAVO GABRIEL

PUBLICACION ESPECIAL - ASOCIACION PALEONTOLOGICA ARGENTINA

ASOCIACIÓN PALEONTOLÓGICA ARGENTINA

Año: 2013 vol. 13 p. 5 - 12

0328-347X

A remarkable feature in the stratigraphy of the Argentine Precordillera is the development of a thick Cambro-Ordovician carbonate platform succession. During the Early Ordovician the carbonates show transition from nearshore shallow environments to distal ramp and slope-basinal black shales. Since late Dapingian times a regional drowning apparently controlled the general facies shift toward graptolitic black shales in more restricted environments. The drowning began in the northern part of the basin and the carbonate factory shut down practically everywhere during the Darriwilian. The Ordovician deposits from the Precordillera basin are related to the development of five major depositional sequences (Astini, 1993; Keller et al., 1998; Cañas, 1999). The San Juan Formation represents the transgressive Lower Ordovician San Juan sequence (Cañas, 1999). The San Juan -Gualcamayo sequence represents the transitional diachronous drowning of the platform (Hünicken, 1985) and the Los Azules sequence constitutes the complete drowning of the platform and the high stand, black shale deposits (Albanesi et al., 1998). However, an unresolved issue is contrasted here: was the drowning continuous until the complete demise of the carbonate platform or did a small regression occur in between these two sedimentary units? We analyzed in detailed four main stratigraphic sections (Fig. 1) where, a two meters thick interval of crinoidal grainstones and rudstones occur at the top of the San Juan Formation: the cerro Viejo, cerro La Chilca and Las Aguaditas sections, as well as the Talacasto and Villicum ranges sections). Other previously studied sections in the Precordillera were also included in the analysis.